In vitro experiments have indicated that calcium binds to specific tandem repeat sequences in DNA, and that Ca++ may therefore be involved in regulating gene transcription. The aim of the present work is to apply analytical electron microscopy to measure nuclear calcium concentrations in rat fetal neurons. Very little information currently exists regarding the total calcium concentrations within nuclei of any cell types. Cultured fetal neurons (17 days and 21 days) were centrifuged into pellets and were vitrified by plunging into liquid ethane at a temperature of -170 degrees C. The blocks were cryomicrotomed, and frozen-hydrated sections were transferred into a high-resolution field-emission STEM. After dehydration, the cryosections were imaged in a dark field to visualize the subcellular structure. X-ray spectra were recorded from regions of euchromatin and heterochromatin. Numbers of cations per phosphate (i.e., per DNA base) were determined. For the 21-day fetal neurons, it was found that fewer than one Ca++ ion binds to 2000 bases in the nuclear heterochromatin.